Process and apparatus for treating water with hydrated lime slurry and for dissolving scale

ABSTRACT

A method and apparatus is provided for treating water with already-slaked lime to arrive at a lime slurry that is in a solution or suspension, and delivering the thus treated water to a separating device which separates grit particles therefrom, to recover a high quality lime/water solution or suspension. An automated system controls the addition of lime and water to a lime mixing vessel. An acid wash system is provided which comprises an automated method and apparatus for removing scale buildup, for delivering an acid wash solution to the lime mixing vessel, the lime slurry holding tank and/or the delivery system, or any of them, thereby dissolving the scale buildup.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.14/836,798, filed Aug. 20, 2015, which is a division of U.S. patentapplication Ser. No. 13/302,312, filed Nov. 22, 2011, which issued asU.S. Pat. No. 9,174,861, on Nov. 3, 2015, which are incorporated hereinin their entireties by reference.

THE PRESENT INVENTION

The present invention is directed to a process and apparatus fortreating waste water, sewage sludge and/or purifying drinking water witha hydrated lime slurry in a mixing vessel, after the lime slakingprocess has already taken place, and a process and apparatus fordissolving scale that forms on internal surfaces of process equipment,including a lime mixing vessel, a holding tank, and an optional gritseparation device, by introducing an acid into the system.

BACKGROUND OF THE INVENTION

Lime has been used in the treatment of sewage sludge to removepathogens, lime stabilization of waste water, pasteurization of sludgeand purification of drinking water by adjusting the pH of the water.Examples of such processes involving the use of lime can be found inU.S. Pat. Nos. 5,013,458; 5,229,011; 5,346,616; 5,401,402; 5,405,536;5,433,844; 5,554,279; 5,618,442; and 7,416,673, which contents areincorporated herein by reference.

Calcium oxide (CaO), is mixed with water (H₂O) to form calcium hydroxide(Ca(OH)₂). The chemical reaction which occurs during such mixing givesoff heat in the form of an exothermic reaction. When this reaction isdone in the presence of excess water it is commonly referred to as limeslaking. This process is accomplished in a device known as a limeslaker. The resulting mixture of Ca(OH)₂ and water is known as a limeslurry and is referred to as a hydrated lime or quicklime or Ca(OH)₂ andhas water trapped in it. There is no further exothermic reaction afterthe lime slaking occurs.

Lime slurries are known to have some unique properties, one of which isan inherent ability to form scale on surfaces which come into contactwith the lime slurry. The formation of scale, or “scaling” can renderthe various delivery systems, such as pipes, troughs, conduits, etc.unusable over time due to the build-up of scale. Such scale build-uprequires frequent cleaning and descaling of the equipment that is usedto handle lime slurries. Cleaning and descaling operations can beconsiderably labor and cost intensive.

Lime slurries also contain a certain amount of inert material that iscommonly referred to as grit. Grit results because the lime contains acertain amount of material other than calcium oxide (CaO). Typically,quicklime is more than 90% pure and contains approximately up to 8%inert material or grit.

The presence of grit in a lime slurry can cause numerous problems,including grit build-up in downstream process chambers or vessels, gritacting to plug or clog nozzles or any orifices through which the slurrypasses, and abrasions caused to treatment equipment and slurry deliveryequipment such as pumps, pipes, valves, etc. because of the abrasivenature of grit particles.

SUMMARY OF THE INVENTION

This invention provides a method of and apparatus for treating waterwith already-slaked lime that is in a solution or suspension anddelivering the same to a separation device which separates gritparticles therefrom, thereby recovering a high-quality lime suspension.An automated system comprising the method steps of this method is alsoan object of this invention.

As used herein, lime slurry is defined to include either lime particlesdissolved in water or a suspension of small lime particles in water, andis also referred to as lime-treated water.

In addition, an acid wash system is provided which comprises anautomated method and apparatus for removing scale build-up in a limemixing vessel, a lime slurry holding tank, and/or a delivery systemcomprising monitoring scale build-up in the lime mixing vessel, the limeslurry holding tank, and/or the delivery system; and at a pre-determinedset point of scale build-up or time, delivering an acid wash solution tothe lime mixing vessel, the lime slurry holding tank, and/or thedelivery system, thereby dissolving the scale build-up.

BRIEF DESCRIPTIONS OF THE DRAWINGS FIGURES

FIG. 1 is a schematic of the system and method for producing highquality lime-treated water according to the invention.

FIG. 1A is a schematic illustration of a portion of FIG. 1, whichrepresents an 10 alternative to the delivery of lime slurry to a dosinglocation, relative to that illustrated in FIG. 1, in which a meteringpump is used in lieu of a pinch valve.

FIG. 2 is a schematic fragmentary view of a multi-point precision dosingarrangement, wherein the slurry from a slurry loop pump deliverslime-treated water to a plurality of dosing locations, which can be atdifferent dosing rates as the slurry is pumped to a grit classifier.

FIG. 2A is an illustration similar to that of FIG. 2, but wherein thedosing locations differ from those of FIG. 2, in that there is not areverse bend in the line between the grit traps and flowmeters.

FIG. 3 is a schematic of the acid wash system in accordance of thisinvention, whereby scale is removed from the various components of thesystem.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

First, it will be noted that in FIG. 1, the lime mixing system and itscomponents are principally illustrated, along with a holding tank andgrit classifier, with various sensors, controls and delivery lines.

In FIG. 3, many of the same components as are illustrated in FIG. 1 arealso illustrated, but wherein FIG. 3 emphasizes the acid wash system,its sensors and controls. It will be understood that the system of thisinvention can be and preferably is a combination of that which isillustrated in each of FIGS. 1 and 3, but that, for simplicity ofpresentation and ease of understanding, FIG. 1 is principally directedto the lime mixing system and related components, whereas FIG. 3 isprincipally directed to the acid wash system and related components.

In FIG. 1 there is illustrated a system that comprises apparatus andmethod to automatically and continuously deliver precise doses of highquality lime/water solution or suspension and in FIG. 2, the doses areshown to be delivered at a plurality of locations.

In the system as illustrated in FIGS. 1 and 2, the operating system iscapable of functioning with little or no operator attention, input, orroutine maintenance.

In FIG. 1, the system is generally designated by the numeral 10. Lime 11is present in a lime storage silo 12 and is delivered via a deliveryauger, conveyor or other delivery device 13 via a suitable delivery line14, to a lime mixing vessel 15. The operation of the delivery device 13may be controlled by a suitable preferably electric motor 16 that inturn, is automatically activated via a control line 17 from aprogrammable logic computer 18.

The lime 11 in the lime storage silo 12 includes grit particles. Thewater that is introduced into the mixing vessel may be at anytemperature that is available to the facility in which the mixing takesplace. The lime is essentially hydrated lime, so no chemical reactionoccurs.

Lines 21 and 22 deliver water into the lime mixing vessel 15 via spraynozzles 24, 25 respectively. Line 23 may deliver water into the limemixing vessel 15 either through spray nozzle(s) or via any deliverymeans, as may be desired.

The water is provided, preferably via line 33. The slurry holding tank28 receives lime slurry from the lime mixing vessel 15, via deliveryline 30.

The water delivered via line 33 to the mixing vessel 15 is via any oflines 21, 22 and 23.

It will be understood that the lime delivery conveyor 13 can be avolumetric feeder device, the control of which is via motor 16 asdictated by the computer 18.

The water that is delivered to the lime mixing vessel 15 via lines 21,22 and/or 23 can be controlled by suitable valves 40, 41, 42 and 43 thatare also controlled from the computer 18, via respective control lines44, 45, 46 and 47.

The lime mixing vessel 15 may sit on a plurality of load cells 48, 50,that in turn support the lime mixing vessel 15 on a floor 53, with theload cells 48, 50 being connected to the computer 18 via signal lines 51and 52, so that the computer 18 can automatically control the inflow ofwater and lime into the lime mixing vessel 15, by controlling the limefeeder motor 16 and the various water supply valves 40-43.

The load cells may be constructed as are the load cells in U.S. Pat. No.7,669,348, or any one or more of U.S. Pat. Nos. 5,770,823; 4,064,744;4,166,997; 4,454,770 and 5,313,022, the complete disclosures of whichare herein incorporated by reference.

It will also be understood that the lime mixing vessel 15 and theholding tank 28 are provided with suitable respective mixers 56, 57,motor driven at 58, 60 respectively.

The system 10 also includes a discharge line 61 from the slurry holdingtank 28, which discharges to the inlet or suction side of a pump 62,which in turn, delivers the slurry via line 63, to a separation device64, that is of the gravity operative grit classifier type. Theseparation device 64 comprises a tank with a sloped auger 65 whichconveys grit up a slope 66, which grit has become separated from theliquid 71 in the tank 64, in that the lower end of the auger 65 isimmersed in the liquid therein, and operates to engage the gritparticles and convey them up the sloped surface to be discharged at 68.The separation device 64 can be either external to or integral with thelime slurry holding tank 28. The auger conveys the grit 70 out thedischarge 68 thereof, into a dumpster 84 or other removable dischargeunit.

Prior to delivering slurry via line 63 to the separation device 64, aportion of the lime-treated water can be extracted at line 76, through avalve 77, through a grit trap 78, then via line 80, through a magneticflowmeter 81 and pinch valve 82, to a dosing location therebeneath, at83.

FIG. 1A illustrates in schematic form, in which lime-treated water isextracted from line 63, via line 76, through a valve 77, through a grittrap 78, and then via line 80 through a magnetic flowmeter 81 and ametering pump 82″ that is used in lieu of the pinch valve 82 of FIG. 1,to a dosing location therebeneath, at 83″. The pump 82″ uses positivedisplacement to push the flow of slurry to the dosing location 83″. Thisarrangement is successfully done when one is dosing to an applicationpoint 83″ that is under pressure, and wherein the pressure is higherthan the pressure in the loop 63, so that the pressure in the loop 63can overcome the pressure at the application point 83″. The particularmetering pump 82″ could be selected in size to overcome the pressure atlocation 83″.

In FIG. 2A, the dosing locations 90 a, 91 a, 92 a, 93 a and 94 a areshown extending in generally straight lines commencing at 76 a, throughvalves 77 a, through grit traps 78 a, through magnetic flowmeters 81 aand pinch valve 82, to their respective 5 locations, as an alternativearrangement to that illustrated in FIG. 2.

In FIGS. 2 and 2A, respective valve 59′ and 59 a can be applied tosubstantially close the dosing loops 63, 63 a to atmosphere, byadjusting the valves 59′ or 59 a to maintain a relatively constantpressure, to create a back pressure from the aging tank 27 of FIG. 1,through delivery line 61, pump 62, delivery line 63, and to the dosingarrangement commencing at line 76 of FIG. 1, or to the dosingarrangements of FIGS. 2 and 2A. In FIG. 1, the valve 59 could,optionally, create a similar back pressure, as may be desired.

The separation device 64 may have one or more baffles in the form oflaminar flow plates 72, 73, 74 and 75, one or more of which may beadjustable, as indicated for the baffle plate 72 between the full lineand phantom line positions thereof, for facilitating separation of gritfrom the lime-treated water in the separation device 64.

The extraction of a portion of the lime-treated water from holding tank28 can be performed more than once by returning the lime-treated waterfrom the separation device 64 via line 67, back to the holding tank 28.

The separation device 64 may be provided with a movable wall andassociated electric or pneumatic activation drives in accordance withU.S. Pat. No. 7,416,673, and the separation device 64 may be constructedin accordance with the separation devices of any of U.S. Pat. No.7,416,673 and U.S. patent publication 2008/0185324 published Aug. 7,2008 and/or published application 2006/0231507 published Oct. 19, 2006,the complete disclosures of which are all herein incorporated byreference. If a movable wall is provided for the separation device 64,such may enable one to vary the cross-sectional surface area of thedevice 64, which will cause an automatic variation in the volume of theliquid contained within the separation device 64. The return of thelime-treated water back to the holding tank 28 via line 67 enablesfurther purification and grit removal.

As described above, the lime mixing vessel 15 can be mounted on one ormore load cells or weighing cells for weighing the contents thereof;namely the lime and water. Load cells are electronic weighing devicesthat add a precise quantity of water to a known quantity of hydratedlime to optimize the lime mixture. The amount of lime and water to beadded into the lime mixing vessel 15 can also be determined byvolumetric or gravimetric means. The lime mixing vessel 15, as describedabove, has a motor equipped mixer 56, which can be controlled remotelyand automatically.

After the lime/water mixing is completed, the lime slurry can be dilutedto a predetermined concentration set point selected preferably withinthe range of about 5%-25%, or in the range of 10% to 20%, utilizing thedirect weight measurement of lime and water via the load cells. Theslurry concentration is within + or −0.5% of the set point (i.e.9.5%-10.5% for a 10% slurry concentration setting).

The lime slurry holding tank 28 can be provided with an access cover atits upper end (not shown). The access cover can be gasketed to containsteam and dust. The bottom of the tank 28 can include a flangeconnection for the slurry pump 62 (not shown). Also, a separate drainopening can be provided for maintenance and cleaning of the lime slurryholding tank 28, also not shown. The motor 60, as with the motor 58 forthe lime mixing vessel can include a speed reducer, which speed reducercan be grease lubricated and directly connected to the mixer shaftassembly.

The pump 62 for delivering lime-treated water from the holding tank 28to the separation device 64 can be selected in size to provide a desiredvelocity through a conduit so that the grit particles and the limeparticles do not settle within the conduit 63. The selected velocity offlow through all conduits of the invention also provides a scouringaction from entrained grit particles, which helps keep the conduits fromclogging due to build-up of scale within the conduits. The pump 62 canalso contain seals that are self-sealing and require no water, as wellas a high pressure switch. The separation device 64 allows for a smallquantity of grit particles to remain in the lime-treated water to assistthe scouring action and prevention of scale build-up.

The lime slurry holding tank 28 gradually and continuously deliverslime-treated water to the separation device 64 where grit is separatedand removed by means of the slurry pump 62 and via the lime slurry loop63.

The lime-treated water is then recycled into the aging tank 28 and runthrough the separation device 64 as many times as desired.

The system can also use fine grit classifier hydraulic separation todeliver a high-quality lime-treated water with minimal grit particles.Considering that the specific gravity of grit is approximately threetimes that of water, 2.65 versus 1.0, grit particles settle at apredetermined rate and adjusting the “rise rate” of the water allows fora precise separation to be made. For example, if grit settles at 3 feetper minute, then the rise rate of water can be set at 2 feet per minute.This hydraulic separation principle is used to keep the grit in the grittrap 78. Dosing of the high quality lime-treated water can also occur atmultiple points along the loop 63.

With reference now to FIG. 2, there is provided a schematic illustrationof the manner in which the lime/water loop 63 between the holding tank28 and grit separator 64 can deliver dosing to any of a plurality oflocations, even at different rates. It will be seen that the deliveryloop 63 receives lime-treated water in the direction of the arrows, anddelivers the same to any of a plurality of dosing locations in amulti-dosing location arrangement. Such dosing locations are indicatedby the numerals 90, 91, 92, 93 and 94. It will be understood that anynumber of such dosing locations may exist on the loop 63.

As with the dosing described above with respect to FIG. 1, each of thedosing locations includes a line 76′, a valve 77′, a grit trap 78′, anexit line 80′, a magnetic flow meter 81′ and a pinch valve 82′.

In the illustration of FIG. 2, the dosing apparatus for location 93 isout of service, because the valve 77′ for that location 93 is in theclosed position, in order to illustrate that not all dosing locationsneed to be operative at any given time. The other dosing locations maydeliver precise dosing at varying rates. For example, one dosinglocation may be delivering 9.5 gallons per minute, for example, atdosing location 94, whereas dosing location 92 may be delivering 10.2gallons per minute, with dosing location 91 delivering 9.7 gallons perminute, and dosing location 90 delivering 10.9 gallons per minute. Itwill be understood that substantial variation may be provided in thedosing rates, depending upon the desires of an operator.

It will be understood that with respect to the above description ofFIGS. 1 and 2, there is provided a method and apparatus for creating alime/water slurry for delivering high quality lime-treated water,preferably in batches, comprising automating the steps of the methodsdescribed above.

This invention also provides an automated system for automatically andcontinuously delivering a dosage of high-quality lime-treated water.

The above-described method can further comprise the step of dissolvingscale buildup in the lime mixing vessel 15 and/or the lime slurryholding tank 28 by performing an acid wash after the lime/water mixing.An example of an acid wash system of this invention is illustrated inFIG. 3.

Referring to FIG. 3, this invention also provides a method for removingscale buildup in a lime/water mixing vessel 15, a lime slurry holdingtank 28, or any delivery system comprising (a) monitoring scale build-upin the lime mixing vessel 15 via monitor 100, the lime slurry holdingtank 28 via monitor 101, and/or the delivery system including theseparation device 64 via the monitor 122; and (b) at a pre-determinedset point of scale buildup or time, delivering an acid wash solution tothe lime mixing vessel 15, the lime slurry holding tank 28 and/or thedelivery system by use of an acid wash pump 102, thereby dissolving thescale build-up.

The acid is placed into the lime mixing vessel 15 through an inlet 103thereof, either manually or automatically. If automatically, thecomputer 18, acting in response to one or more scale build-up monitors100, 101 and 122, through control line 89, discharges acid from the acidsupply 99 through supply line 109 to inlet 103. The acid enters the limemixing vessel 15 above the liquid level 105 thereof. Water is deliveredvia line 23, as needed, into the lime mixing vessel 15, depending uponthe operation of a valve 106, controlled either manually, or from thecomputer 18 via control line 107. As acid enters the lime mixing vessel15, it is extracted after mixing with liquids therein, via line 108,upon opening of the valve 110 at the bottom thereof, which valve 110 canbe controlled from the computer 18 via control line 111, and then thepump 102, activated via control line 112 from the computer 18, ormanually, can deliver the acid/water solution back into the lime mixingvessel 15 via line 113, 115, lines 21 and/or 22 to the spray nozzles 114that are disposed above the liquid level 105, in order to expose theentirety of the lime mixing vessel 15 to the acid wash. The acid rinseline 113 also delivers the acid rinse to the holding tank 28 via lines115 and 116, to enter the holding tank 28 via one or more spray nozzles117 therein, also located above the liquid level 118 thereof. Similarly,the acid wash is delivered to the separation device 64 by means of thepump 102, through line 113, line 115 and line 120, to enter theseparation device 64 through one or a plurality of spray nozzles 121.Then, the generally continuous recirculation of the acid rinse fromholding tank 28, via pump 62, delivery line 63, and back into theseparation tank 64, then back into the holding tank 28 via line 67,continues the recirculation of the acid wash. The monitors 100 in any ofthe mixing vessel 15, 101 in the holding tank 28, and 122 in theseparation device 64, can continually monitor the level of scalebuild-up in those vessels, and can communicate via signal lines 123, 124and 125, respectively, to signal to the computer 18 when it is necessaryto provide the acid rinse described above to the various equipment inthe system. Then, the acid wash can be implemented manually orautomatically.

When the monitoring step and delivering step are performed in the limemixing vessel 15, this method can further comprise the step ofdischarging the acid wash solution from the lime mixing vessel 15 to thelime slurry holding tank 28 and any other part of the system.

The acid wash system of the invention can be applied to the lime mixingvessel 15, lime slurry holding tank 28, or any other part of the system.Spray nozzles can be positioned to be above the liquid level so that thenozzles delivering lime and water can be cleaned during the acid washcycle. At least two spray nozzles can be installed in a vessel tomaximize the coverage of the acid wash spray. The spray nozzles 114 canbe eliminated, and water could be delivered directly into the limemixing vessel 15 via lines 21, 22, in order to flood the lime mixingvessel 15 with the acid solution. The water entering the lime mixingvessel 15 in order to accomplish the acid wash can be under sufficientpressure to suit varied condition, and can be applied with variations inpressure, to provide a scouring system along with the solvent or acid,to aid in the cleaning of the entire system. In this regard, the entiresystem can include not only any of the lime mixing vessel, holding tankand grit separation device, but also the various piping, including thelines 30, 61, the pump 62, 63, and dosing sub-systems, including theextraction line 76, valve 77, grit trap 78, line 80, magnetic flowmeter81 and pinch valve 82 of FIG. 1, and, alternatively, the line 63,extraction line 76, valve 77, grit trap 78, line 80, magnetic flowmeter81 and metering pump 82″ of FIG. 1A. After the acid wash cycle, either abatch of lime slurry or water can be used to flush the system to cleanit. This acid wash system enables the system to run smoothly andprovides adequate flow of the slurry in the system's conduits. If theacid wash cycle is to be automatically triggered, it can be set to startat certain times, or after a certain number of slaking cycles, or bymeasuring the build-up of scale by use of the load cells instead of anyof the monitors 100, 101 and 122. In the case of using the load cells,the system utilizes a system wherein the weight of scale is calculatedand when a critical mass is reached, the acid wash cycle is triggered.

The automatic acid washing system of this invention is thus able to usea solvent such as an acid where the lime mixing vessel, the slurryholding tank, and the remainder of the delivery system, including thegrit separation device can be neutralized and whereby any scale build-upcan be dissolved. The quantity of scale that is present in each batchbeing treated in the apparatus of this invention, being monitored by themonitors 100, 101 and 122 can be recorded in the computer 18, and whenthe level of scale build-up reaches a predetermined set point, oralternatively, on a periodic basis, a rinse cycle can be initiatedwhereby acid can be added to the lime mixing vessel 15, either manuallythrough the access opening 103 thereof, or automatically from theabove-described acid feed into the access opening 103, and whereby wateris added to the lime mixing vessel 15 as described above, at a levelabove the liquid levels in the lime mixing vessel, the holding tank, andthe separation device, in such a manner that the entire contents of allof the equipment are totally rinsed by feeding the acid/water throughspray nozzles throughout the system, as described above. The rinsesolution in the lime mixing vessel apparatus 15 can be discharged intothe slurry holding tank 28 and the process can be repeated before therinse solution is introduced into the delivery system. The deliverysystem can be cleaned by dosing a predetermined, variable and precisequantity of solvent solution into the potable water treatment process.The flow rate or rates of dosing, as addressed in the description abovewith respect to FIG. 2, can be the same, or variable, and such can bemonitored and such information can be fed back to the computer control18, or to a manual control, if desired. Thus, while it is known thatlime produces scaling in the apparatus that handles the same, the scalecan be removed to increase the reliability of the system, by measuringscale build-up and removing the same upon the scale build-up reaching apredetermined level, or periodically based upon a timed repeatscale-removal system. The system also sprays the acid/water solutionthroughout the equipment above the liquid level in the various limemixing vessel 15, holding tank 28, and separation device 64, asdescribed above, with the acid/water solution also being run through theloop 63, and dosed at any of various controlled rates into the watersupply.

It will be understood that various modifications may be made to thesystem, including its method steps and apparatus, and its operation, allwithin the spirit and scope of the invention as defined in the appendedclaims.

What is claimed is:
 1. A process of treating water with lime comprisingthe steps of: (a) providing calcium hydroxide and grit particles to alime mixing vessel; (b) providing water to the lime mixing vessel toform a lime slurry therein; (c) delivering the lime slurry from the limemixing vessel to a slurry holding tank; (d) delivering the lime slurryfrom the slurry holding tank to a separation device for separating gritparticles from the lime slurry; and (e) including the step of thendelivering the lime slurry to at least one dosing location.
 2. Theprocess of claim 1, further including the step of separating the gritparticles from the lime slurry by gravity in the separation device. 3.The process of claim 1, wherein the lime slurry is delivered to aplurality of dosing locations at different dosing rates for at leastsome of the locations.
 4. The process of claim 1, wherein the process isdone seriatim, in batches.
 5. The process of claim 1, wherein theseparation device includes baffle plate(s), and further including thestep of separating the grit particles from the lime slurry, wherein thelime slurry is flowed over the baffle plate(s) to effect hydraulicseparation of the grit particles from the lime slurry.
 6. The process ofclaim 5, including the step of adjusting the orientation of the baffleplate(s).
 7. The process of claim 1, further including controlling theprovision of calcium hydroxide and grit, as well as of water, to themixing vessel via a programmable logic computer.